PNG Proposed eXIF chunk, draft 20170207

File: png-proposed-eXIF-chunk-20170207.html

Status of this Memo

This document is an informal draft of the PNG development group.

Comments on this document can be sent to the PNG specification maintainers at

Distribution of this memo is unlimited.

Initial proposals were in the png-mng-misc mailing list, January 2017.

At present, the latest version of this document is available on the World Wide Web from


Copyright © 2017 Glenn Randers-Pehrson

Permission is granted to copy and distribute this document for any purpose and without charge, provided that the copyright notice and this notice are preserved, and that any substantive changes or deletions from the original are clearly marked.


This document describes a special-purpose chunk type that has been proposed for use in various PNG (Portable Network Graphics) and related multi-image applications. It has not yet been approved for registration by the PNG developers, and therefore is experimental and its format is subject to change. The proposed chunk is "eXIF" (an unregistered name that can be used in test implementations is "exIF"). Caution: the third letter in the chunk name is an uppercase "i", not a lowercase "L".

PNG exIF (raw Exif) proposal

It is proposed to add the following section to the document "Extensions to the PNG 1.2 Specification, Version 1.4.0"

3.7. eXIF Exif (Exchangeable Image File) profile

The four-byte chunk type field contains the decimal values
    101 88 70 102 (ASCII "eXIF")

The data segment of the eXIF chunk contains an Exif profile in the format specified in "4.7.2 Interoperability Structure of APP1 in Compressed Data" of [CIPA DC-008-2016] except that the JPEG APP1 marker, length, and the "Exif ID code" described in 4.7.2(C), i.e., "Exif", NULL, padding byte, are not included. It begins with either "II" or "MM", depending upon the byte order used.

At most one eXIF chunk shall occur in a PNG datastream. There are no ordering constraints upon the position of the eXIF beyond those imposed by the PNG specification, i.e., if present, the eXIF chunk may appear anywhere between the IHDR and IEND chunks except between IDAT chunks.

The eXIF chunk contains metadata concerning the original image data. This data may not apply or may no longer apply to the PNG image data. To avoid potential conflicts between data in the eXIF chunk and in other PNG chunks, it is declared that only those Exif tags that contain historical information such as data about the camera, picture-taking conditions, and geographical location, and that do not affect subsequent image processing have any meaning. Such tags are listed in the Exif Specification (CIPA DC-008, Exchangeable image file format for digital still cameras) Annex E, Paragraph 3, with the notation "should be copied unchanged".

Encoders may write all known Exif tags in the eXIF chunk; however, any other tags which might be present in the eXIF chunk shall be ignored by decoders, and tags containing information relevant only to the original JPEG or TIFF encoding shall be ignored by decoders.

If a thumbnail image (or multiple thumbnail images) is present, there is no expectation that such thumbnails have been updated if the main image was changed, or that they have not been updated.

See ISO 12234 (the XMP standard):

CIPA DC-010-2012, Exif 2.3 Metadata for XMP. Available at:

CIPA DC-008-translation-2012, Exchangeable image file format for digital still cameras: Exif Version 2.3. Available at:

Description of the Exif file format, (based on Exif Version 2.1), TsuruZoh Tachibanaya, May 28, 1999,

eXIF Security considerations

This chunk does not introduce addtional security issues.


Contributors to discussion on the png-mng-misc list


End of PNG eXIF Chunk Proposal. Expires 7 September 2017.